malloc

tonightelvis 2008-01-11 06:41:18

一个实现malloc的例子，出错了

my-malloc.h
# include <stdlib.h>
typedef long Align; /* for alignment to long boundary */
union header { /* block header: */
struct {
union header *next; /* next block if on Free list */
unsigned int size; /* size of this block */
} s;
Align x; /* force alignment of blocks */
};
typedef union header Header;

# define NALLOC 10 /* minimum #units to request */
static Header* morecore(unsigned int nu);
void* Malloc(unsigned int nbytes);
void Free(void *ap);

my-malloc.c
# include <unistd.h>
# include "my-malloc.h"

static Header base; /* empy list to get started */
static Header* free_list = NULL; /* start of free list */

//定义Malloc函数：分配一块内存空间
void* Malloc(unsigned int nbytes)
{
Header *p, *prev;
unsigned int nunits;
nunits = (nbytes + sizeof(Header) -1) / sizeof(Header) + 1;
if ((prev=free_list) == NULL) // no free list yet //
{ // 没有节点
base.s.next=free_list=prev=&base;
base.s.size=0;
}
for (p=prev->s.next;;prev=p,p=p->s.next)
{
if (p->s.size>=nunits){ // big enough
if (p->s.size==nunits) // exactly
prev->s.next=p->s.next;
else {
p->s.size-=nunits;
p+=p->s.size;
p->s.size=nunits;
}
free_list=prev;
return(void*)(p+1);
}
if(p==free_list) /* wrapped around Free list */
if((p=morecore(nunits))==NULL) /* 如果系统不提供新的内存空间，则返回NULL */
return NULL; /* none left */
} /* endfor */
}

// 向系统申请足够大的一块空间
static Header* morecore(unsigned int nu){
char *cp;
Header *up;

if(nu<NALLOC)
nu=NALLOC;
cp=sbrk(nu * sizeof(Header));
printf("sbrk:%X--%X\n",cp,cp+nu * sizeof(Header));
if(cp==(char*)-1) // 没有空间则返回NULL
return NULL;
up=(Header*)cp;
up->s.size=nu;
Free(up+1);
return free_list;
}

//put block ap in Free list
void Free(void* ap)
{
Header *bp,*p;
bp=(Header*)ap-1; /* point to block header */
for(p=free_list;!(bp>p&&bp<p->s.next);p=p->s.next)
if(p>=p->s.next&&(bp>p||bp<p->s.next))
break; // freed block at start or end of arena
if(bp+bp->s.size==p->s.next){ // 如果新申请的空闲空间和另一空闲空间邻接则合并
bp->s.size+=p->s.next->s.size;
bp->s.next=p->s.next->s.next;
}
else
bp->s.next=p->s.next;
if(p+p->s.size==bp){
p->s.size+=bp->s.size;
p->s.next=bp->s.next;
}
else
p->s.next=bp;
free_list=p;
}

// 打印分配给用户的空闲块链表
void printlist(void)
{
Header *p;
int i=0;
printf("base:%X, base.next:%X, base.next.next:%X\n", &base,base.s.next, base.s.next->s.next,free_list);
for(p=&base; p->s.next!=free_list; p=p->s.next)
{
i++;
printf("block%d,size=%d",i,p->s.size);
if (p>free_list)
printf("used!\n");
else
printf("free!\n");
}
}

int main()
{
char *p[200];
int i;

for (i=0;i<20;i++)
{
p[i]=(char *) Malloc(8);
printf("malloc %d, %X\n", i, p[i]);
printlist();
}
for (i=19; i>=0; i--)
{
Free(p[i]);
printf("free %d \n", i);
printlist();
}
return 0;
}

test.c
# include "my-malloc.h"
void main(void)
{
/* print_list(); */
char *p[200] ;
int i;

for (i = 0; i < 20; i++) {
p[i] = (char *) Malloc(8) ;
printf("malloc %d, %X\n", i, p[i] ) ;
print_list() ;
}

for (i = 19; i >= 0; i - -) {
Free( p[i] ) ;
printf("free %d \n" , i) ;
print_list() ;
}
}

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